In the early days of telecommunication, a copper wire medium was used to carry a single information channel. Because the greatest proportion of cost is in the materials and construction of the physical link, telephony engineers have developed ways to pack multiple channels onto a single physical link. Frequency division multiplexing (FDM) and time division multiplexing (TDM) have been devised to multiplex multiple streams of analog and pulse code modulation (PCM) digital signals, respectively, into one. For digital signals, the TDM hierarchy is DSO through DS4, where a DSO is a single 0.064 Mbits/sec channel and a DS4 is 4,032 message channels (DSOs) multiplexed together.
A similar TDM scheme is used in international telephone systems based on 32-channel format. The international digital systems, based on International Telecommunication Union CCITT's G.700 Series Recommendations, are commonly called E1 or CEPT-1. The E1 signals are based on blocks of 32 channels or time slots, of which time slot 0 and time slot 16 typically are used for control and signaling, respectively.
Although both the U.S. and international digital systems provide for the multiplexing of DSO or E1 signals to form higher rate signals, greater efficiency of the physical telecommunications link is desired. The capability to transport a higher concentration of voice and data channels advantageously decreases the number of physical links and further lower the toll exacted for connecting the calls.